1. Field of the Invention
This invention relates to the field of spread-spectrum clock circuits, specifically to spread-spectrum clock circuits used to reduce Electro Magnetic Interference (EMI) in electronic systems.
2. Description of the Related Art
Spread Spectrum Clocking (xe2x80x9cSSCxe2x80x9d) is used to spread the EMI energy of a system across a wide bandwidth. The spread of EMI energy reduces the overall EMI emission of the system. In SSC, the stable clock source of the system is frequency modulated which causes the frequency to deviate (e.g., between 0.25% and 5%). SSC design is used for EMI compliance of the system.
During compliance testing of computing systems, EMI is measured with a receiver with a fixed bandwidth, usually 120 KHz. When two or more emission sources lie within this bandwidth, the emission from each source is added linearly. For example, if two separate emission sources of equal amplitude are separated by less than 120 KHz in frequency, the receiver detects a signal that is twice the amplitude (e.g., 6 dB greater) than that of either source.
Generally, complex computing systems includes multiple emission sources (e.g., a rack of servers or the like). When multiple systems are measured, the base frequency of each system can fall within the receiver""s bandwidth (e.g., 120 KHz). If a single frequency is generated by each emission source within the system, the combined emission can be the sum of all the emission generated by each source. Thus, a computing system with multiple emission sources can fail the over compliance test. Therefore, a method and apparatus is needed to reduce EMI emission from multiple sources within a system.
In one embodiment of the present invention, a method of reducing electromagnetic interference in a system is described. The method includes generating a spread-spectrum master clock, generating a plurality of phase-offset spread-spectrum slave clocks from the spread-spectrum master clock and using the plurality of phase-offset spread-spectrum slave clocks for at least one electromagnetic interference source. Each one of the phase-offset spread-spectrum slave clocks has a different phase-offset. The different phase-offsets are generated using a phase delays. In one embodiment, the phase delays are predetermined. In another embodiment, the phase delays are dynamically calculated.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.